1. Field of the Invention
The present invention generally relates to an operation amplifier, and more particularly, to an operation amplifier for improving a slew rate.
2. Description of Related Art
FIG. 1A illustrates a conventional source driver. The source driver includes a Gamma generator 110, an interface circuit 120 and a Digital-to-Analog Converter (DAC) 130 for receiving a digital data Din to drive a panel (not shown). The Gamma generator 110 provides a plurality of Gamma reference voltages to the DAC 130. The DAC 130 selects one of the Gamma reference voltages respectively in accordance with the digital data outputted from the interface circuit 120. The DAC 130 outputs the selected Gamma reference voltage(s) for driving the panel (not shown). In one embodiment, the Gamma generator 110 generates three sets of Gamma reference voltages, and the three sets respectively correspond to R, G and B data. The first set of the Gamma reference voltages includes red Gamma reference voltages Gr0, Gr1, Grn; the second set of Gamma reference voltages includes green Gamma reference voltages Gg0, Gg1, . . . , Ggn; the third set of Gamma reference voltages includes blue Gamma reference voltages Gb0, Gb1, . . . , Gbn.
FIG. 1B illustrates the output stage of the Gamma generator 110 in the source driver of FIG. 1A. The output stage of the Gamma generator 110 includes OP amplifiers OPA0, OPA1, . . . , OPAn. Each OP amplifier is used as a buffer that sequentially receives the corresponding red, green and blue Gamma reference voltages for outputting to the DAC 130. Specifically, the buffer OPA0 receives the Gamma reference voltages (Gr0, Gg0, Gb0); the buffer OPA1 receives the Gamma reference voltages (Gr1, Gg1, Gb1); the buffer OPAn receives the Gamma reference voltages (Grn, Ggn, Gbn).
FIG. 1C illustrates a circuit of a conventional OP amplifier. The OP amplifier OPAn includes an input stage 111 and an output stage 112. The input stage 111 may respectively receive input voltages Vin1 through Vinm of different magnitudes, and the output stage 112 correspondingly generates different output voltages. For example, when the input stage 111 receives an input voltage Vin1, the output stage 112 outputs a corresponding output voltage. Then, when the input stage 111 receives an input voltage Vin2, the output stage 112 outputs a corresponding output voltage. Then, when the input stage 111 receives an input voltage Vin3, the output stage 112 outputs another output voltage. However, if the time intervals of the input voltages Vin1 through Vin3 when inputting to the input stage 111 are relatively short, and a slew rate of the operation amplifier OPAn is not high enough, the output voltages outputted from the output stage 112 are unlikely to be accurate as expected.
As well understood, the slew rate of an OP amplifier OPAn is determined by a ratio between a bias current I at the input stage 111 and a compensation capacitance Cc, that is I/Cc. As such, when the compensation capacitance Cc is fixed, increasing the bias current I at the input stage 111 is the only way to improve the slew rate. However, simply increasing the bias current I unfortunately increases the power consumption of the OP amplifier OPAn.